1. Field of the Invention
This invention relates to a method and circuit for producing multiple intensity or gray scale operation in a gaseous discharge display panel.
2. Description of the Prior Art
Multiple cell gas discharge display and/or memory panels of one particular type with which the present invention is concerned are characterized by an ionizable gaseous medium, usually a mixture of at least two gases at an appropriate gas pressure, in a thin gas chamber or space between a pair of opposed dielectric charge storage members. The dielectric charge storage members are typically backed by arrays of electrodes which are appropriately oriented so as to define a plurality of discrete gas discharge units or cells.
In some prior art panels, the discharge cells are additionally defined by surrounding or confining physical structure such as apertures in perforated glass plates and the like so as to be physically isolated relative to other cells. In either case, with or without the confining physical structure, electronic charges (electrons and ions) produced upon ionization of the gas volume of a selected discharge cell, when proper alternating operating potentials are applied to selected conductors thereof, are collected upon the surfaces of the dielectric at specifically defined locations. These charges constitute an electrical field opposing the electrical field which created them so as to terminate the discharge for the remainder of the half cycle and aid in the initiation of a discharge on a succeeding opposite half cycle of applied voltage, such charges as are stored constituting an electrical memory.
Thus, the dielectric layers prevent the passage of substantial conductive current from the conductor members to the gaseous medium and also serve as collecting surfaces for ionized gaseous medium charges (electrons and ions) during the alternate half cycles of the a.c. operating potentials, Such charges collect first on one elemental or discrete dielectric surface area and then on an opposing elemental or discrete dielectric surface area on alternate half cycles to constitute an electrical memory.
An example of a panel structure containing non-physically isolated or open discharge cells is disclosed in U.S. Pat. No. 3,499,167 (incorporated herein by reference) issued to Theodore C. Baker, et al.
An example of a panel containing physically isolated cells is disclosed in the article by D.L. Bitzer and H. G. Slottow entitled "The Plasma Display Panel- A Digitally Addressable Display With Inherent Memory," Proceeding of the Fall Joint Computer Conference, IEEE, San Francisco, Cal., Nov., 1966, pages 541-547 and also in U.S. Pat. No. 3,559,190 (incorporated herein by reference) issued to D.L. Bitzer et al.
In the construction of the panel, a continuous volume of ionizable gas is confined between a pair of dielectric surfaces backed by electrode arrays typically forming matrix elements. The two electrode arrays may be orthogonally related sets of parallel lines (but any other configuration of electrode arrays may be used). The two arrays define at their intersections a plurality of opposed pairs of charge storage areas on the surfaces of the dielectric bounding or confining the gas. Thus, for an electrode matrix having H rows and C columns the number of elemental or discrete charge storage areas will be twice the number of elemental discharge cells.
In addition to the matrix configuration, the electrode arrays of the display device may be shaped otherwise. Accordingly, while the typical electrode arrangement is of the crossed grid type as discussed herein, it is likewise apparent that where a maximal variety of two dimensional display patterns is not necessary, as where specific standardized visual shapes (e.g., numerals, letters words, etc.) are to be formed and image resolution is not critical, the electrode may be shaped accordingly (e.g., a segmented digit display).
Gas discharge display panels are normally operated in the bistable memory mode. In this mode there are two intensity states, on and off. A number of techniques have been proposed for obtaining multiple intensity or gray scale operation in these panels. The basic five categories of operation are:
1. Spatial gray scale -- More than one discharge site is used per resolution element and the intensity is varied by varying the number of sites turned on. Such a system is disclosed in U.S. Pat. No. 3,845,243 issued to Larry J. Schmersal et al. The discharge panel is divided into a number of discharge regions each having the same number of storage and discharge areas or sites which number is equal to the number of gray levels to be produced. In synchronism with the line by line scanning of an image, a plurality of storage means are loaded for each picture element scanned with a number of information bits which corresponds to the intensity of the picture element scanned. After the scanning of each image line, the stored information corresponding to that line is read out to excite the appropriate number of discharge areas in each discharge region. When viewed from a distance a gray scale is produced.
2. Time modulation of bistable states - Discharge sites are turned on and off at varying times within a refresh period and the intensity is varied by controlling the relative on time. Such a system is disclosed in U.S. Pat. No. 3,863,023 issued to Larry J. Schmersal et al. A number of gray level ranges are defined and an equal number of memory planes are provided, each memory plane having the same number of storage areas as the number of storage and discharge areas in the display panel. An image field is scanned with a vidicon tube and the elements of the signal corresponding to the picture elements are digitized according to the gray level range in which they fall and are fed in sequence to the memory plane corresponding to that gray level. The contents of each memory is then read out to excite the display panel beginning with the memory corresponding to the brightest gray level range and ending with the memory corresponding to the dimmest gray level range. Due to the storage characteristics of the panel, the bits read from the memory corresponding to the brightest gray level range excite corresponding discharge areas of the display panel for a longer period of time than the bits from the memory planes corresponding to the dimmer gray level ranges and the resulting display picture has graduation of gray coresponding to the original scanned image.
3. Multistable states - The cell characteristics and sustain voltage wave form are exploited to give more than one kind of stable discharge sequence resulting in several different levels of intensity. Such operation is limited in the number of intensity levels achieved.
4. Ordered dither - An information processing technique whereby the intensity of each picture element is compared with a positionally dependent threshold giving a spatially varying intensity due to the changing density of bistable discharge sites in the on state.
5. Stacked panels - Superimposing transparent panels one behind the other such that the intensity variations are determined by how many sites at a given location are in the on state.
It has been shown that all of these techniques are capable in principle of providing variable intensity. However, they all have some practical limitations for certain applications. Method 1 reduces the effective resolution of the display panel. Method 2 requires an extensive peripheral electronic memory. Method 3 has a very limited number of intensity levels (for example, three). Method 4 has so far produced only medium quality continuous tone pictures. Method 5 is cumbersome and possibly expensive to implement.